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chapter eight section fifty two defining compression, Dithering, and Color
palettes.
again i'm going to use recycle.psd that can be downloaded from lisatime.
this is just a little recycle image.
.jpg, .gif, and .png have different compression options in the
save for web
and devices dialog box so we're going to look at those different options.
i'm going to file. im going to save for web and devices.
in the last section we set the
second window to the .gif format.
we're going to choose again-- it doesnt remember our last setting so i'm
going to say the lower because there are limited colors in this,
the .gif thirty-two dithered.
then i'm going to choose the third window and im going to change that
to the--
actually, i changed my mind. im going to go to the second window, change that to j-peg
medium.
go to the third window, change that to the .gif
thirty-two dithered.
choose the
fourth window and change that to .png-8
one twenty eight dithered.
now, in that selection
the j-peg image that you're looking at, remember, can contain sixteen milllion
colors.
it has no options for compression, dithering and number of colors
because it's lossy compression is controlled through the quality settings.
so if we have the j-peg selected you'll notice that the panel at
the right looks different than it does when you're looking at the .gif and the .png
formats.
there's no color table because it can include sixteen million colors.
the only
method you have of adjusting the compression is to reduce the quality.
so when we choose j-peg medium we get a thirty percent
and you have the ability
to slide that up just a bit if you chose to, to say forty percent.
or um...
you can choose one of the j-peg high
or low options.
of course if you increase that quality you're going to increase the file size
slightly.
so that's the only option you have for j-peg in controlling the compression,
the dithering, and the color palettes.
when you go to the .gif format-- i chose the third window, the .gif format,
and i look at the panel on the right.
you see i do have several more options available.
when i look at that format i have
two hundred and fifty six colors available to me,
but i don't need two hundred and fifty six; i specified a color table of
thirty-two colors
and you'll see the colors that are going to be used. the colors that exist within
the color table
is sometimes referred to as the color palette.
i did choose the dithered option.
dither settings
scatter different colored pixels in an image to make it appear as though there
are intermediate colors and images
with a limited color palette.
you can also set the number of colors to be used in the image.
the lower the number, the lower the file size. and the .gif also offers a lossy
setting - how much detail is lost and
because, by default, no detail is lost, but you could allow it lose detail to
make the file size smaller.
and a web snap option
which would have forced the colors to the web-safe color palette.
so i chose the .gif thirty-two dithered but as you can see this is actually
just a green image.
I could lower that number
down to two
and still retain the colors, but it gets edgy. two is not enough within the color table.
i have an edgy-looking image.
so i might try something like a sixteen,
and a sixteen is fine with me. i could go with a sixteen and that's 1.504k
as compared to the thirty-two we were using a moment ago, which is 1.67.
if we were to adjust that all the way to the available number of colors, 256,
it's 2.114k and there actually is no reason to use that many
colors, no one's going to notice the difference.
i think it looks
just about as good at 16 as it did at 256 because of
the limited colors in use.
the png-8 format would be very similar to that.
the ping has a color table, it has a number of colors - right now it's set to
128. let's also take it to 16.
you'll notice very few color changes.
it has the web snap option where you can snap to the next available color format.
so you can dither
just as you can with the .gif format and you can also set the number colors. again,
lowering the number of colors reduces the file size.
something we haven't talked about
is the png-24 format.
the png-24 format may contain sixteen million colors and is more similar to
the j-peg format.
i'm going to click upon the first window and in the first window i'm going
to drop the arrow down and not choose the original,
but the png-24.
and the png-24, again, will have no color table because it can have sixteen
million colors.
the difference between the two,
the .jpeg and
the .png,
is that the .png can contain areas of transparency.
and it's 3.32 in size.
if i were the one saving this image,
i would look at all of these options and make a decision based upon the smallest
file size
and the best quality.
i'm not going to save it this time; i'm going to hit cancel.
and that concludes section 52.